The invention relates generally to power-driven conveyors and more particularly to spiral conveyors in which a conveyor belt is driven in a helical path around a rotating drive tower.
Conveyor belts are often used to convey articles, such as foodstuffs and other materials, through cooled or heated environments. Spiral conveyors, in which a conveyor belt follows a helical path winding around a central tower, drum, or cage, are used in freezers and ovens to provide a long conveying path with a small footprint.
Some helical conveyors are constructed with a helical track supported on a central non-rotating tower. The conveyor belt is driven around the helical track by drive sprockets at a single location outside the helical path. The maximum tension in the belt, which occurs just ahead of its engagement with the drive sprockets, can be quite high for such a long belt. To reduce the maximum belt tension, overdrive spiral conveyor systems are used. In these overdrive systems, the conveyor belt is driven by frictional contact between the inside edge of the belt and the faster-rotating outer surface of the rotating drum about which the belt is helically wrapped. Because the belt is driven along the entire helical path, the maximum belt tension is decreased. But some tension is still needed for effective frictional engagement between the drum and the belt edge. Furthermore, the frictional engagement causes wear in the belt edge and the outer drum surfaces. Because a large portion of the rotational energy required to drive the drum is lost to friction, the motor and power requirements can be quite high. And, because overdrive systems are sensitive to friction between the outside of the drum and the inside edge of the belt, the proper settings of tension and overdrive vary from installation to installation.
Positively driven spiral systems, in which drive structure on the outside of a rotating cage engages structure on the inside of a conveyor belt, have been used to overcome some of the shortcomings of overdrive systems. Because there is positive engagement between regularly spaced drive structure on the cage and regularly spaced edge structure on the inside edge of the belt, there is no slip as in overdrive systems. No additional tensioning is needed and frictional losses are less. But one problem with positively driven spiral systems is in cleanly engaging the belt with and disengaging it from the drive structure on the cage. Another problem is that the belt loses its driving force as it exits the drive drum. In conventional spiral conveyors a take-up roller or sprocket downstream of the drum is operated at a constant speed. If the speeds of the take-up motor and the drum's motor are matched to ensure that the belt speed remains constant, operation will be problem-free. But changes in belt temperature, such in freezer or proofer applications, cause the belt to shrink and expand, which affects the timing of the belt from drum to take-up. When the speeds of a drum's motor and the take-up motor are fixed, the take-up can't adjust to the timing changes and will either pull too hard or not hard enough.